# jQuery Library
## Including jQuery
### Download and link the file
```html
<head>
<script src="jquery-3.2.1.min.js"></script>
</head>
```
### Use a CDN
```html
<head>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.5.1/jquery.min.js"></script>
</head>
<!-- OR -->
<head>
<script src="https://ajax.aspnetcdn.com/ajax/jQuery/jquery-3.5.1.min.js"></script>
</head>
```
### What is a CDN
A **content delivery network** or **content distribution network** (CDN) is a large distributed system of servers deployed in multiple data centers across the Internet.
The goal of a CDN is to serve content to end-users with high availability and high performance.
CDNs serve a large fraction of the Internet content today, including web objects (text, graphics and scripts), downloadable objects (media files, software, documents), applications (e-commerce, portals), live streaming media, on-demand streaming media, and social networks.
## HTML Manipulation
### [Finding DOM elements](https://api.jquery.com/category/selectors/)
```js
$('tag');
$("#id");
$(".class");
```
### Manipulating DOM elements
```js
$("p").addClass("special");
```
```html
<!-- before -->
<p>Welcome to jQuery<p>
<!-- after -->
<p class="special">Welcome to jQuery<p>
```
### Reading Elements
```html
<a id="yahoo" href="http://www.yahoo.com" style="font-size:20px;">Yahoo!</a>
```
```js
// find it & store it
var link = $('a#yahoo');
// get info about it
link.html(); // 'Yahoo!'
link.attr('href'); // 'http://www.yahoo.com'
link.css('font-size'); // '20px
```
### Modifying Elements
```js
// jQuery
$('a').html('Yahoo!');
$('a').attr('href', 'http://www.yahoo.com');
$('a').css({'color': 'purple'});
```
```html
<!-- before -->
<a href="http://www.google.com">Google</a>
<!-- after -->
<a href="http://www.yahoo.com" style="color:purple">Yahoo</a>
```
### Create, Store, Manipulate and inject
```js
let paragraph = $('<p class="intro">Welcome<p>'); // creat and store element
paragraph.css('propery', 'value'); // manipulate element
$("body").append(paragraph); // inject in DOM
```
### Regular DOM Nodes to jQuery Objects
```js
var paragraphs = $('p'); // an array
var aParagraph = paragraphs[0]; // a regular DOM node
var $aParagraph = $(paragraphs[0]); // a jQuery Object
// can also use loops
for(var i = 0; i < paragraphs.length; i++) {
var element = paragraphs[i];
var paragraph = $(element);
paragraph.html(paragraph.html() + ' WOW!');
}
```
## [Events](https://api.jquery.com/category/events/)
```js
var onButtonClick = function() {
console.log('clicked!');
}
// with named callback & .on
$('button').on('click', onButtonClick);
// with anonymous callback & .on
$('button').on('click', function(){
console.log('clicked!');
});
// with .click & named callback
$('button').click(onButtonClick);
```
### Preventing Default Event
```js
$('selector').on('event', function(event) {
event.preventDefault();
// custom logic
});
```
## Plugins
In the HTML, add a `<script>` ag that hotlinks to the CDN or source file:
```html
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery-validate/1.17.0/jquery.validate.min.js"><script>
```
In the JavaScript call the jQuery puging on the DOM:
```js
$("form").validate();
```
**NOTE**: always link to the [minified](https://developers.google.com/speed/docs/insights/MinifyResources) js files.
## More jQuery
### Patters & Anti-Patterns
```js
// Pattern: name variables with $var
$myVar =$('#myNode');
// Pattern: store references to callback functions
var callback = function(argument){
// do something cool
};
$(document).on('click', 'p', myCallback);
// Anti-pattern: anonymous functions
$(document).on('click', 'p', function(argument){
// do something anonymous
});
```
### Chaining
```js
banner.css('color', 'red');
banner.html('Welcome!');
banner.show();
// same as:
banner.css('color', 'red').html('Welcome!').show();
// same as:
banner.css('color', 'red')
.html('Welcome!')
.show();
```
### DOM Readiness
DOM manipulation and event binding doesn’t work if the `<script>` is in the `<head>`
```js
$(document).ready(function() {
// the DOM is fully loaded
});
$(window).on('load', function(){
// the DOM and all assets (including images) are loaded
});
```
## AJAX (jQuery `1.5`+)
```js
$.ajax({
method: 'POST',
url: 'some.php',
data: { name: 'John', location: 'Boston'}
})
.done(function(msg){alert('Data Saved: '+ msg);})
.fail(function(jqXHR, textStatus){alert('Request failed: '+ textStatus);});
```